A reflecting surface may be provided with periodic resonant apertures or slots. Such a reflecting surface will reflect incident electromagnetic energy in all frequency bands except for the band of frequencies associated with the geometry, i.e., the size and shape, of the resonant slots and the periodic spacing of the slots on the reflecting surface. In principle, these resonant slots act as perfect band-pass filter circuits. The centerband frequency of radiation transmitted through these slots is a function of the periodic spacing of the resonant slots. The size and shape of the slots determines the width and shape of the band of frequencies transmitted through the slots. The resonant apertures, or resonant slots, can be made effective for all polarizations of incident electromagnetic energy (and therefore said to be polarization insensitive) in one of several geometric configurations. They may be of the "cross"-type, which covers both horizontal and vertical polarizations of incident electromagnetic energy, or of the "Y"-type, which also covers both horizontal and vertical polarizations. The radiation incident upon the resonant aperture in the band of frequencies associated with the resonant slot geometry is partially transmitted, partially back-scattered and partially reflected. The visibility of the reflecting surface is greatly reduced in the pertinent band of frequencies, because very little energy is reflected.
However, it is often desirable to reduce the visibility of a large flat or curved reflecting surface to signals emitted by a distant transmitter, while at the same time providing a good reflecting surface for signals emitted from a nearby transmitter in the same frequency band as the distant transmitter.
It is possible to render the resonant slot fully reflecting in the relevant band of frequencies by connecting a switching diode across the center of the resonant slot and rendering the diode conductive by the application of a direct current bias voltage across the diode from an external power source, as disclosed in U.S. Pat. No. 4,314,249 to Onoe, which is incorporated herein by reference.
It is also desirable to render the resonant slots fully reflecting without employing conductance means dependent upon external power sources, i.e., power sources other than the power source provided by the electromagnetic energy incident upon the reflecting surface. Power sources external to the power provided by the electromagnetic energy incident upon the reflecting surface require additional transmission apparatus and introduce additional requirements for initiating diode switching.